【作者】 伍来智；

【导师】 陈军；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2010， 博士

【摘要】 随着现代工业和科学技术的发展,各行业钢制构件正朝着巨型化、高性能方向发展。而晶粒细化是唯一能同时提高钢铁材料强度和韧性的强化机制。因此,超细晶粒钢研究已成为国际上高性能结构材料研究领域中备受关注的热点之一。本文结合国家自然科学基金课题“结构钢热塑性成形过程中组织细化机理”,采用实验研究和理论分析相结合的方法,对SPHC钢在热塑性成形过程中的奥氏体再结晶行为及应变诱导铁素体相变过程进行了系统研究。分析了热变形工艺参数在组织细化中的作用,并提出了低碳结构钢热塑性成形过程中晶粒细化的数学模型,探讨了组织细化的机理,为新一代超细晶钢生产提供理论指导。论文的主要研究结果如下:采用单道次热压缩实验,对SPHC钢在热塑性成形过程中的动态再结晶规律进行了研究。显微组织分析表明:降低变形温度和增加应变速率,可以导致动态再结晶晶粒尺寸的减小。通过分析动态再结晶的应力-应变曲线,采用三次多项式拟合θ-σ和ln（θ）-σ曲线计算得到了发生动态再结晶的临界条件的数学模型。计算得到了动态再结晶激活能为299.4 kJ/mol。建立了SPHC钢动态再结晶的动力学模型和晶粒尺寸模型。采用双道次热压缩实验,对SPHC钢高温变形后的静态再结晶及亚动态再结晶规律进行了研究。研究表明:高的变形温度、大应变量、高应变速率及细小的初始奥氏体晶粒都有利于静态再结晶的发生,静态再结晶后的晶粒尺寸随着应变量的增大和初始晶粒尺寸的减小而减小。亚动态再结晶主要受应变速率和变形温度的影响,随着应变速率的增大,亚动态再结晶过程加快,亚动态再结晶的晶粒尺寸减小;随着变形温度的降低,亚动态再结晶进行变得困难,而亚动态再结晶晶粒尺寸减小。计算得到了SPHC钢的静态再结晶激活能和亚动态再结晶激活能分别为284.1kJ/mol、195.6 kJ/mol,建立了各自的再结晶动力学模型和晶粒尺寸模型。在Gleeble 1500型热模拟机上通过不同的实验工艺,研究了热变形工艺参数对应变诱导铁素体相变的影响规律。结果表明:随着应变量的增大和变形温度的降低,应变诱导相变产生的铁素体数量增多、晶粒尺寸细化;随着应变速率的增大,铁素体数量和晶粒尺寸都减小;铁素体晶粒的数量随初始奥氏体晶粒尺寸的减小而增加。应变诱导铁素体相变机制为扩散相变机制。在引入一套计算奥氏体变形储存能的方法基础上,以超组元模型为热力学基础,利用修正的KRC模型,开发了多组元结构钢的应变诱导铁素体相变热力学计算模型,并分析了变形储存能对应变诱导铁素体相变热力学的影响。计算表明:奥氏体的变形储存能随着变形温度的降低和应变速率的升高而增加;应变诱导铁素体相变的驱动力随着变形储存能的增加而升高,其影响效果与温度范围有关,越靠近A_e3温度,影响效果就越明显;应变诱导铁素体相变平衡温度A_e3随着变形储存能的增加,即温度的降低或应变速率的提高而升高。本文首次通过分析应变诱导铁素体相变的应力-应变曲线,采用三次多项式拟合θ-σ和ln （θ）-σ曲线计算得到了发生应变诱导铁素体相变的临界条件的数学模型。通过实验分析,本文认为应变诱导铁素体相变过程是一个不断形核的过程。因此,本文首次提出了计算应变诱导铁素体相变时间的模型,在Cahan晶界形核的等温相变动力学理论基础上,以单一的“形核长大”机制提出了应变诱导铁素体相变的动力学模型。利用该模型对一些实验结果的模拟计算证明,本模型所预测热变形工艺参数对应变诱导相变形成的铁素体体积分数、铁素体平均晶粒尺寸的影响与实际情况相符,充分证明了本文模型的可用性。更多还原

【Abstract】 With the development of modern industry and scientific technology, steel structural parts have been evolved towards the trend of giant and high performance. Grain refining strengthening has been proved to be the optimum way that can improve both the strength and the toughness. Therefore, the study of ultra-fine steel is becoming one of the most heated topics in the field of structural materials. The works of this dissertation are carried out integrating with the National Natural Science Foundation of China-Microstructure refinement mechanism for hot forming process of structural steel. The austenite recrystallizaiton behavior and strain induced ferrite transformation (SIFT) during hot forming for low carbon structural steel SPHC are investigated. The effects of hot working processing parameters on microstructure refinement are investigated also, and the integrated mathematical models for prediction of microstructure refinement are developed. The research results are summarized as the following:The dynamic recrystallization (DRX) behaviors of SPHC steel during hot deformation are investigated using single pass compression test. The metallographic analysis shows that grain size of DRX decreases with increasing strain rate or decreasing deformation temperature. The critical condition models for the initiation of DRX are built by analyzing stress-strain curves of DRX, and fittingθ-σcurves and ln (θ)-σcurves with cubic polynomial. The calculated activation energy of DRX is 299.4 kJ/mol, and the dynamic recrystallization kinetics and grain size models are established.The static recrystallization (SRX) and metadynamic recrystallizaiton (MDRX) behaviors of SPHC steel are investigated using double pass compression test. The results show that the volume fractions of SRX rapidly increase with the increase of the deformation temperature, strain, and strain rate. Also, the volume fractions increase with the decrease of initial austenite grain sizes. The grain size after SRX is affected mainly by strain and initial austenite grain size. The grain size of SRX decreases with increasing strain or decreasing initial austenite grain size. The metadynamic recrystallizaiton is affected mainly by strain rate and deformation temperature. With increasing strain rate, the MDRX progress accelerates and the grain size of MDRX decrease. As the deformation temperature decreases, the MDRX progress becomes difficult, but the grain size of MDRX decreases. The calculated activation energies of SRX and MDRX are 284.1kJ/mol and 195.6 kJ/mol respectively. Their kinetics and grain size models are built, respectively.The effects of hot working processing parameters on strain induced ferrite (SIF) transformation behavior are investigated on Gleeble 1500 hot simulator. The results show that SIF amount increases and SIF grain size decreases with increasing strain or decreasing deformation temperature. SIF amount and SIF grain size decreases with increasing strain rate. SIF amount increases with decreasing the initial austenite grain size. The mechanism of SIFT is mainly a diffusional transformation.A method for calculating deformation stored energy of austenite is proposed. On the thermodynamic basis of superelement mode and modified KRC mode, the thermodynamics parameters of multicomponent steels for SIFT are calculated, and the effects of deformation stored energy on the thermodynamics parameters are analyzed. The results of the calculations show that the value of deformation stored energy increases with lowering the deformation temperature or increasing strain rate. Deformation stored energy enhance the driving force of SIFT, which is relevant to deformation temperature, and the nearer the equilibrium temperature of austenite to ferrite transformation Ae3, the more obvious impact effect. The equilibrium temperature of austenite to ferrite transformation Ae3 rises with increasing deformation stored energy.The critical condition models for the initiation of SIFT are established by analyzing stress-strain curves of SIFT and fittingθ-σcurves and ln(θ)-σcurves with cubic polynomial.By the experimental analysis, nucleation is consider to be a dominant course during SIFT. Therefore, the model of SIFT is suggested on the view that SIFT is a single course of nucleation and growth. A method for calculating SIFT time is proposed, and SIFT kinetics model is suggested based on Cahn’s isothermal transformation kinetics theory of grain boundary nucleation and growth mechanism. The simulations of some experiments prove that calculated results agree with the measured results in the effect of hot working processing parameters on transformed fraction and ferrite grain size. So the SIFT model of present paper is valuable for analyzing the process of SIFT.更多还原